1. Field of the Invention
The present invention relates in general to electrostatic copy mediums and a method for producing positive and negative latent electrostatic charge patterns. More specifically it relates to copy mediums and a method for producing latent electrostatic charge patterns utilizing piezoelectric and photoconductive materials.
2. Description of the Prior Art
A number of patents disclose processes for producing charge patterns utilizing multi-layer copy mediums having an insulative layer, an electrically conductive layer and a photoconductive intermediate layer. Such disclosures are taught in U.S. Pats. to Ohta et al, No. 3,677,711, Makino et al, No. 3,719,481 and Matsumoto, No. 3,775,104. These patents teach initial charging of copy mediums by applications of ions from external corona discharge power sources, and also teach the usage of such power sources for applying ions to copy mediums during image formation.
Another known copy medium and process is disclosed in U.S. Pat. No. 3,713,822 issued to Kiess. The medium consists of a layer of crystalline photoconductive-pyroelectric compound on an electrically conductive layer. To produce a copy by use of such a medium in accordance with the teachings of the Kiess patent, first the photoconductive-pyroelectric compound is heated in the dark to develop an initial positive electrostatic charge on one surface of the compound and an initial negative electrostatic charge on the opposite surface. Then, the charged compound is exposed to a light image which converts the exposed image areas from a low conductivity to a high conductivity. The negative and positive charges in the exposed areas combine thus reducing surface charge and voltage potential in the exposed areas. The result is an electrostatic charge pattern on the exposed surface of the photoconductive-pyroelectric compound. However, since the electrostatic charge pattern representative of the image is formed on the photoconductive-pyroelectric layer, it must be immediately developed to avoid dispersion of charge as the result of leakage through the photoconductive-pyroelectric layer.
My co-pending U.S. application Ser. No. 385,849 filed Aug. 6, 1973 now U.S. Pat. No. 3,992,204 and entitled "Method and Medium for Producing Electrostatic Charge Patterns" discloses a multilayer copy medium having a pyroelectric insulative layer, an electrically conductive layer and an intermediate photoconductive layer. A process is also taught in such application for producing a latent electrostatic charge pattern by use of the disclosed medium, which process is performed by first subjecting the pyroelectric layer to a temperature change to develop an initial positive electrostatic charge on one surface of the pyroelectric layer and an initial negative electrostatic charge on the opposite surface of the pyroelectric layer. Then, at least a portion of the charge from the surface of the pyroelectric layer not in contact with the photoconductive layer is transferred to the electrically conductive layer such that there are opposite polarity charges and a voltage potential across the photoconductive layer. Last, exposure of the photoconductive layer to a light image converts the exposed image areas of the photoconductive layer from a low conductivity to a high conductivity, thus, permitting the negative and positive charges of the exposed areas to combine.
Further disclosed in my above described co-pending application is an improved method and medium for producing an image on the surface of an insulative layer by means of an image transfer from the surface of the photoconductor to the insulative layer.
The present invention differs from the known copy mediums described above by employing a piezoelectric layer to provide an insulative layer that can readily be charged by stressing without the need for external charging units.